Fuel cells offer tremendous potential for improving energy efficiency and reducing greenhouse gas emissions, with potential applications ranging from transport to stationary power generation. In terms of the latter application, the Solid Oxide Fuel Cell (SOFC) (operating temperature 500-1000○C) has attracted widespread interest, due to many benefits in terms of fuel flexibility and efficiency [1]. Traditionally oxide ion conducting electrolytes have been favoured for such SOFCs, although proton conducting electrolytes have also been attracting growing interest [3]. In addition, such proton conducting electrolytes are required for steam electrolysers, attracting growing interest for the generation of hydrogen utilising renewable energy sources. In these cases, as protons are the conducting ion, the hydrogen is produced separate from the supplied water, which is an important advantage.

The vast majority of research in this area has focused on the electrolytes, with a range of structures being shown to exhibit proton conductivity [2,3]. The preferred systems are the perovskite-type materials, which are doped with acceptor ions to introduce oxide ion vacancies [2]. These systems typically display high oxide ion conductivity at elevated temperatures in dry conditions, while in wet atmospheres, the conductivity is enhanced through the incorporation of water into the oxide ion vacancies, leading to the presence of proton defects.

These protonic defects account for the proton conductivity observed in doped BaCeO3, BaZrO3 and related systems. However, a key factor that is now limiting progress in the proton conducting ceramic electrolyte fuel cell/electrolyser area is the electrodes employed, especially on the air/oxygen side. Currently researchers are principally employing electrode materials that have been developed for use with oxide ion conducting electrolytes. While some, such as Ba0.5Sr0.5Co0.8Fe0.2O3-y, Pr2NiO4, have shown respectable performance, the performance still significantly lags behind the performance of such electrodes with oxide ion conducting electrolytes. Moreover, Ba0.5Sr0.5Co0.8Fe0.2O3-y is a metastable material, and on long term operation under fuel cell operating conditions, a slow degradation in performance occurs. Clearly there is therefore the need to develop bespoke systems that are tailored for the use with proton conducting electrolytes.

Research Project
As noted above, research into electrode materials for use with proton conducting ceramic fuel cells has focused on materials that have been developed for use with oxide ion conducting electrolytes, which has led to performances that significantly lag behind those observed in conjunction with oxide ion conducting electrolytes.

If one considers the key aspects which are needed for cathodes for use with such proton conducting ceramic electrolytes, factors to consider include:
1. The compatibility (chemical and thermal) with the electrolyte
2. The need for proton conduction in the electrode
3. The stability in fuel cell operation